At the present time, there are a wide variety of different laser types or devices known, which are employed for a variety of uses. One suggested use is for communication purposes. Lasers offer two potential advantages, namely the availability of a large oscillating band width and hence the possibility of transmitting a large amount of information, and secondly the small wavelength of a laser gives a very small degree of divergence, approximately 10,000 times smaller than microwaves. However, the beams provided by lasers are strongly attenuated in the atmosphere under conditions of poor visibility. Nonetheless, in optical fiber systems, there is the possibility of transmitting a large amount of information.
There is also the problem of encoding the information into the laser beam. Various techniques have been proposed for modulating the laser beam. For transmitting information or data digitally, it has been proposed to provide some means for switching the laser on and off. This however, can lead to frequency chirping, since the laser is not running continuously. Ideally, a laser should be running continuously, so that it reaches a steady temperature etc., and then the frequency is steady.
Another technique is to couple the output of the laser through an electro-optic device capable of varying the degree of polarization given to the light. An example of this is a Pockels cell, provided with crossed polarizers at either end. The induced birefringence of the crystal varies with the applied electric field. Thus, by switching the electric field, one can switch the polarization of the light leaving the crystal. The effect of the polarizers is then to determine whether the beam is transmitted through or not. This thus enables the light beam to be modulated. However, such crystals do require a very large voltage to be applied to achieve the necessary polarization effects. Typically, voltages of the order of hundreds of volts have to be applied, and even then the crystals are of substantial dimensions. In modern communications equipment where compact integrated circuit components are common operating at low voltages, such an arrangement is unacceptable.
U.S. Pat. No. 4,498,179 to Wayne et al shows a somewhat similar type of arrangement. Here, a modulator is provided in a coupled cavity and is controlled so as to determine whether radiation is transmitted out of the coupled cavity or not. Here, a large voltage would be necessary to operate the modulator. It is also noteworthy that the gain cell itself is provided with Brewster windows to make it heavily anisotropic, whereby it operates in a selected polarization mode. Another well known technique for controlling the operation of a laser, to achieve a pulsed operation which can be used for communication purposes is Q-switching. Effectively, one controls the characteristics of the resonant cavity so that it is switched from a state in which laser action cannot occur, to a state in which laser action can occur. Assuming the gain medium has been pumped to achieve a large population inversion, the stored energy is released as a short burst of radiation.
Reeder U.S. Pat. No. 4,740,986 is an example of a laser resonator that relies upon Q-switching. It includes an electro-optics crystal, which is a Kerr or Pockels cell. This has to provide a quarter-wave effect, and again a large voltage would be necessary to achieve this.
Conventional teaching in the field of lasers is that it is impossible to control the polarization of a quasi-isotropic laser by feedback. Experiments with various lasers have resulted in chaotic or unstable behaviour, which current theories, based on intensity competition, have been unable to explain. It is currently a widely held belief that attempt of any sort to control feedback into a quasi-isotropic laser will merely result in this chaotic behaviour and should be avoided. Thus, many lasers are deliberately designed to be anisotropic, e.g. by the use of Brewster windows, so that the polarization of the radiation is carefully controlled. Clearly, once a laser is made anisotropic, it is not possible to cause the plane of a polarization to switch within the laser itself. Alternatively, it has been common to design laser devices, to eliminate any feedback or at least reduce it to the lowest level possible.
More recently, various semiconductor lasers have been developed. Generally, these are strongly anisotropic. Typically, the actual resonant cavity formed in the semiconductor has the configuration of a wave guide, which prevents any quasi-isotropic behaviour. Nonetheless, attempts have been made to control the polarization mode in a semiconductor laser.
An example can be found in U.S. Pat. No. 4,549,300 (Mitsuhashi et al). Here, adjacent to the semiconductor that provides the gain medium, there is located a controlling element with lenses on either side and a mirror. It is noteworthy that the ends of the semiconductor are coated with antireflection coatings, to reduce the reflectance to below 0.01. In other words, the controlling element, lenses etc. are effectively located with the gain medium in a single resonant cavity, rather than being in a separate section coupled to the main resonant cavity. It is noted that owing to the wave guide configuration of the semiconductor, the directions of polarization at which the laser output can normally assume are limited to the two directions parallel and perpendicular to the plane of the PN junction. By effectively providing in the resonant cavity a device that controls the plane of polarization, the overall device does indeed control the direction of polarization. In another embodiment, a lithium niobate crystal is used to control the polarization. It is noted that voltages as high as a kilovolt are necessary to cause the polarization to switch from one mode to another.
An earlier paper by the inventors, R.E. Mueller and B. Aissaoui, entitled Competition Effects in the Polarization of Light in a Quasi-Isotropic Laser (Journal of the Optical Society of America, Volume 4, No. 8, August 1987, page 1276) reported on experiments involving polarization competition in a quasi-isotropic helium neon laser. In one experiment, the different polarization modes yielded a crenellated line shape. In a second experiment, the internal anisotropies are determined by an inclined internal etalon, and dips and peaks were observed in the intensity of the laser. However, at that time, it had not been determined what level of feedback was necessary to cause the switching or what level was necessary for stable switching between the different modes. As detailed below, the speed at which switching could occur and a variable hysteresis effect in the switching behaviour were also totally unknown. It was not realized that in fact a small change in the feedback was necessary and could be effected by a small, compact modifier of intensity, phase or other parameter.
There is another semiconductor laser, where it has been found that some lasers produced according to a certain design show an instability between the two possible polarization modes. This is exploited by varying the current supplied. At one current level, the device operates at one polarization mode, and switches to the other polarization mode at a different current. This effect is not at all understood, and the frequency chirping problem is not entirely eliminated. The variation in current causes variations in the power and hence the frequency chirping.
What is desirable is a method of modulating a laser beam, so that digital information can be encoded onto the laser beam at a high bit rate. Preferably, the technique should be capable of being implemented in a semiconductor laser, using small, compact components operating at low voltages. This would then enable a small compact device to be made. The encoding technique should have fast switching to enable the high bit rate to be achieved. Further, the laser should preferably operate continuously, to avoid any frequency chirping effects, etc.